The present invention relates to a method for processing waste elastomeric material to recover the material and optionally to de the elastomeric material so as to recover materials such as carbon and volatile hydrocarbon products. The method of the invention is particularly suitable for recovering materials from vehicle tires.
The present invention will be described with particular reference to the treatment of used vehicle tires, however it will be appreciated that the methods and apparatus described herein may also be suitable for the treatment of other articles containing elastomeric materials, such as conveyor belts, and no limitation is intended thereby.
Vehicle tires are typically formed from synthetic and natural rubber materials together with carbon black, plasticizers together with steel reinforcing wires, metal beads at the inner diameter of the tire and fibers formed from nylon or polyester.
Used vehicle tires are commonly disposed of by dumping to landfill. However, dumps of old tires are undesirable for environmental reasons. They take up large amount of space and can be a fire hazard. Also, in view of the costs of manufacture of rubber it would be desirable to be able to recycle the rubber in used tires. Methods which have been proposed to dispose of used tires include simply slicing the tires to reduce the large volume of tire dumps. Used tires may also be ground to produce rubber crumb which may be used in the production of rubber flooring, roads, sports surfaces and other rubber products. Typically, the steel wires and beads are magnetically separated from the crumb during granulation and the fiber separated by flotation systems. Generally, the tires are subjected to an initial slicing or shredding step with the steel and fiber intact. Slicing and shredding of the steel requires additional energy and leads to high wear and tear on slicing and shredding blades.
Another approach has been to carbonize the tires to produce breakdown the rubber to products such as carbon black and fuel oil. Generally, the tires are carbonized whole and the steel is separated afterwards. A disadvantage of heating whole tires is that the carbon and fuel oil, end products can be contaminated by wire and breakdown products of the fibers.
Separation of rubber from reinforcement materials in vehicle tires may be facilitated by first softening the rubber with a solvent followed by subjecting the tire to shear conditions to separate the rubber from the reinforcing materials. However a disadvantage of softening the rubber with solvents is that large volumes of solvents are required. This adds to costs together with the associated health, safety and environmental hazards associated with handling and ultimately disposing of large amounts of solvents. An example of such a process has been described in U.S. Pat. No. 5,316,224 in which material from waste tires was soaked in vats containing solvent for between about 5 to about 6 hours.
Attempts have also been made to reclaim carbon and volatile products from waste rubber by heating the rubber in the absence of oxygen to high temperatures in a microwave. A disadvantage of this process is that although rubber absorbs microwave energy it is a poor conductor. This results in uneven heating of the rubber. This uneven heating can have an adverse effect on the quality of the final product as it may be contaminated by combustion products or products resulting from incomplete carbonization.
It is therefore an object of the present invention to provide a method and apparatus for recovering an elastomeric material from an article containing that material or to provide the public with a useful or commercial choice.
According to a first broad from of the invention, there is provided a method of recovering elastomeric material from an article containing the material, the method including softening the elastomeric material by contacting the article with a fluid comprising an oil in admixture with solvents to soften the elastomeric material and subjecting the softened material to a shearing force to recover the elastomeric material.
Articles containing such elastomeric material suitably include vehicle tires, conveyor belts, rubberized fabrics, and elastomeric materials reinforced with metal, wires, filaments and the like.
The article containing the elastomeric material may be required to be precut or shredded to a processable size. Thus, vehicle tires are debeaded and sliced or coarsely shredded by any conventional shredding apparatus.
Suitable elastomeric materials include those capable of being softened by absorbing liquids into the elastomeric material. These materials may include synthetic or natural rubbers, modified rubbers, vulcanized rubbers, neoprenes, isoprenes, compositions of natural or synthetic rubbers, homopolymers or copolymers of conjugated diene hydrocarbons, homopolymers or copolymers or chloroprene, carboxylated rubbers, halogeneated rubbers, silicones, ABS elastomers, EP and EPT rubbers, cross-linked, graft, block or interpenetrating elastomers.
Suitably, the solvent is selected from hydrocarbons, nitrohydrocarbons, alcohols, ethers, ketones, esters, glycols and glycol ethers, cycloalkyl alcohols, esters and ketones, chlorinated hydrocarbons, cyclic ethers and aldehydes and mixtures thereof.
Preferred solvents in these classes include benzene, toluene, xylene, tetrohydronaphthalene, decahydronaphtalene, dipentene, petroleum liquids, naphtha liquids, nitropropane, methyl alcohol, ethyl alcohol, N-propyl alcohol, N-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, amyl alcohol, benzyl alcohol, diacetone, diethyl ether, diisopropyl ether, acetone, methyl isobutyl ketone, methyl acetate, ethyl acetate, N-butyl acetate, amyl acetate, hexyl acetate, amyl formate, ethyl lactate, butyl glycollate, methyl benzoate, butyl stearate, dimethylphthalate, dibutylphthalate, dibutylsebacate, methylabietate, ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol ethyl ether acetate, ethylene glycol monobutyl ether, ethylene glycol ethyl ether acetate, ethylene glycol monobutyl ether, diethylene glycol, diethylene glycol monoethyl ether, propylene glycol, cyclohexanol, cyclohexanol acetate, cyclohexanone, methyl cyclohexanone, methylene dichloride, chloroform, carbon tetrachloride, dichloroethane, perchloroethane, dichloroethylene, trichloroethylene, perchloroethylene, mono chlorobenzene, dichloroethylether, 1,1,2,-trichlorotrifluroethane, dioxane and furfural.
Preferred solvents include toluene and xylene used alone or in mixture with each other or with other liquids. Other preferred liquids include Shellsol A, Shellsol 1021 and water. The preferred liquids may be mixed with inert liquids of high boiling point such as halogenated hydrocarbons.
Small proportions (5-11%) of inert hydrocarbons such as trichloroethylene, 1,1,1-trichloroethylene or carbon tetrachloride may also be added.
The solvent may also include waste solvents from industries such as the automotive and paint industries. It has been surprisingly found that impurities in these waste solvents does not adversely affect the ability of the liquid mixture to soften the elastomeric materials. Typical waste solvents include a mixture of aromatic solvents such as toluene, xylene and benzene.
The oil may include any suitable oil or mixture thereof. Suitable oils include engine oil, grease, coal oil, fuel oil, paraffin oil, mineral oil and oils derived from plants or animals. An especially preferred oil is that obtained as a product of destructive distillation of an elastomeric material or as a byproduct of carbonization of the tires. Typically such an oil, comprises limonene, other saturated hydrocarbons and aromatic compounds.
Typically the fluid mixture comprises between about 50 to about 90vol % oil and preferably between about 60 to about 80vol %.
In the method of the invention, the article containing the elastomeric material is contacted with the fluid for a time sufficient to soften the material. The material is typically softened to a degree whereby the subsequent shearing step may be carried out efficiently. By way of example, rubber from a vehicle tire typically has a shore hardness of about 60. It is desirable that this shore harness is reduced to between about 25 to about 35 by softening in the fluid.
The present inventor has surprisingly and unexpectedly found that by using a fluid comprising a mixture of a solvent and an oil that the amount of fluid required to soften the material to the desired degree can be significantly reduced compared with the amount of fluid required when using solvent alone. For example, the present inventor has observed that by using a prior art process such as that described in U.S. Pat. No. 5,316,224 rubber from a used vehicle tire must absorb between about 30 to about 50% of its weight of solvent before reaching a desired degree of softness. On the other hand, by using the method of the present invention it may be possible to reduce the amount of fluid required to soften rubber in a vehicle tire to less than about 30% by weight of the rubber.
The oil distilled from tires is an especially preferred medium to carry the chemical compounds into the rubber, weakening the chemical bonds by the softening process. This compound I have named xe2x80x9cSoloilxe2x80x9d. Whilst not wishing to be bound by theory, it is believed that the xe2x80x9cSoloilxe2x80x9d and other suitable oils, when entered into the molecular structure of the rubber (natural/synthetic, styrene/butadiene, chlorinated cyclized/semicyclized and/or any other rubber compounds) elongates the bonds, weakening them, thus lowering the resistance of rubber to mechanical forces with much reduced energy requirements. The solvent component of the Soloil acting on the rubber does not dissolve it, but simply enlarges the molecular structure until the solvents are extracted again. It has been found that if a volatile oil is mixed with volatile solvents, the boiling point of the mixture is reduced. It is believed that the azeotropic blend has an increased solvent power as the viscosity of the mixture is increased, thus the use of oil reduces the need to use large quantities of solvents. Obviously, when waste solvents/chemicals are plentiful, the tire recycling system can assist in a responsible and controlled use of the product (rather than dumping) then this invention accommodates that activity.
The following table relates to the viscosity of the blends of workable proportions at 15xc2x0 C. It""s obvious that a number of various recipes can be applied.
Compound B is a waste industrial solvent which contains at least some xylene and/or toluene.
As in the first invention as described in U.S. Pat. No. 5,316,224, I selected a number of solvents which are non-polar (the soak-attraction time to the structure of the rubber was rather long), with the improvements by mixing selected solvents with oil they become polar, subsequently the attraction forces are increased without the need of raising the temperature or pressure or both.
In the case of vehicle tires, it is preferred that the tires are debeaded and sliced or chopped before being subjected to soaking. Apparatus and methods for debeading, slicing and chopping used vehicle tires are known and commercially available.
In an especially preferred embodiment, there is provided a method of softening an article containing an elastomeric material, the method including placing the article in a closed vessel containing a fluid which can absorb into and soften the material, whereby the vessel has an inlet at a first end and an outlet at a second end and the article passes through the vessel from the inlet to, the outlet for a time sufficient to soften the elastomeric material.
Typically, the article passes from the inlet to the outlet under the influence of gravity, in which case the vessel is inclined such that the inlet is above the outlet. In this case, the articles may be automatically agitated as they cascade downwards towards the outlet. Typically, vessel is a pipe or conduit (which I named a cascading duct system). The residence time in the vessel may be dependent upon the length of the vessel. A typical residence time for vehicle tires is up to about 4 hours. A typical vessel used for treating used vehicle tires is a conduit between about 500 m to about 2,000 m in length. The vessel need not be linear and typically has a repeating S or spiral shape.
If desired, the vessel may be subjected to heating and/or pressure.
It is preferred that the fluid is an oil solvent mixture as described above. However, a fluid such as a solvent mixture as previously described in U.S. Pat. No. 5,316,224 may also be used. However, in this case, the residence time in the vessel may need to be increased.
Typically when the articles reach the outlet the articles are separated from the fluid and the excess fluid is filtered and recycled.
According to a further broad from of the invention there is provided an apparatus for treating an article containing an elastomeric material, the apparatus including a closed vessel containing a fluid which can absorb into and soften the elastomeric material, the vessel having a first end having an inlet and a second end having an outlet, whereby the article passes from the inlet to the outlet for a time sufficient to soften the elastomeric material.
Typically the apparatus further includes means for recycling excess fluid from the outlet to the inlet. Typically the apparatus also includes collection means for collecting and recycling vapors from volatilized fluid in the apparatus.
After softening, the article is subjected to a shearing force, which can separate the elastomeric material in the article from non-elastomeric material such as wire and fibre tire reinforcement. One suitable shearing means is that described in U.S. Pat. No. 5,316,224.
An especially preferred device for shearing the softened material includes at least two pairs of counter rotating rollers, each roller having a spirally configured flight, whereby the pairs of rollers are mounted one above the other and the gap between the rollers in a respective pair is largest for the uppermost pair of rollers and in use material to be sheared passes sequentially from the upper to the lowermost pair of rollers.
The rollers compress and twist the softened elastomeric material which results in separation of the elastomeric material from any non-elastomeric material in the article. Where the article is a vehicle tire, the rollers separate the rubber from the wire and fibers of the tire reinforcement. The number of rollers is typically dependent on the volume and type of the elastomeric material. For processing rubber tires, the shearing device typically includes four pairs of counter rotating rollers. Typically, the gap between the uppermost pair of rollers is between about 6 to about 7 mm. The next three pairs of rollers typically have gaps of between about 4 to about 5 mm, between about 2 to about 3 mm and less than 1 mm respectively.
It has been surprisingly discovered that by providing at least two pairs of rollers with different gaps between the rollers in a respective pair that a more efficient action for separating elastomeric material from non-elastomeric material such as wire reinforcement can be achieved.
Where the article includes wire reinforcement, the device typically further includes magnetic separation means for separating any metal from the elastomeric material.
Rollers having screw flights typically have a directional action which transfers material along the flight. In some cases, this may lead to excess material being carried to one end of the roller where it is unable to be processed effectively. It is therefore preferred that the rollers are mounted at an angle to the horizontal such that the direction of rotation pushes the material in an uphill direction. In this way gravity is used to at least partially counteract the directional movement of the roller.
Alternatively, or in addition to mounting the rollers at an angle, the rollers may be manufactured such that each half of a roller has a forward and backward directional movement. This can be accomplished by changing the flight angle midway along the roller.
The typical measurement of any two working augers is: pitch (90 mm-120 mm), gradient (42xc2x0-48xc2x0), depth (50 mm-85 mm), length (1500 mm), diameter (400 mm-600 mm) and working surface area of approximately between about 1.0 and about 1.8 m2, preferably about 1.4 m2 per set of augers.
Typically, the shearing device further includes means for removing any of the fluid mixture, which may be volatilized during the shearing process. Typically, fumes are removed by vacuum and collected in liquid form. This liquid may be recycled to the soaking step.
The softened elastomeric material may be collected and part or all of the absorbed fluid may be removed. Removal of the absorbed fluid may be carried out by a variety of suitable techniques including rotary kiln drying, radiation drying such as microwave drying and/or squeeze drying. Where the elastomeric material has been softened with an oil/solvent mixture and the material to be subjected to carbonization by microwave irradiation, it is preferred that at least some of the oil remains in the material. This will be referred to in further detail below.
If desired the elastomeric material which has been separated from the non-elastomeric material is subjected to size reduction. Softening of the elastomeric material typically results in swelling which increases the size of a piece of material. Thus, slicing or grinding can be facilitated by the increase in size as compared to grinding or slicing a shrunk piece of rubber. Typically, the material is reduced to particles having an average size of below about 100 mesh.
The elastomeric material may be ground in grinders which are known in the art. However, it is preferred that a grinder further includes a screw auger located within an elongated housing. As the auger rotates inside the housing it turns cutting blades which cut the rubber. The screw auger drives the material forward and compresses it against perforated plates. Rotating blades slice the material and when the material reaches a certain particle size, the particles can escape through the holes in the perforated plate. As a result of compression and friction generated during this process heat is generated. It is therefore desirable that the grinder further includes a cooling system. A typical cooling system includes an external cooling jacket which may be built into the housing and/or internal cooling at the center of the auger. Typically any fumes generated during granulation are removed by vacuum and may be collected and recycled to the soaking step.
The ground elastomeric material may then be treated to at least partially remove the fluid. Typically, the ground material is heated either under vacuum or ambient pressure. The liquids are typically collected and may be recycled to the soaking step. The ground elastomeric material is typically suitable for use in floortiles, sports surfaces, roads and other uses to which conventional types of ground rubber may be applied.
The ground product is particularly suited to degradation by carbonization produce commercial products such as fuel oil and carbon black. Preferably, the ground product is carbonized using microwave energy. A suitable source of microwave energy may be a microwave reactor as described in U.S. Pat. No. 5,316,224.
Typically, the microwave reactor includes a chamber with a first inlet for introducing the elastomeric material into the chamber, an outlet for removal of solid degraded products, a second inlet to allow microwave radiation to enter the chamber and a second outlet for removal of volatile materials from the chamber.
Typically, the chamber is rotatable to facilitate mixing of the material so as to provide a more even energy distribution. It has been observed that by providing uniform heating the percentage of carbon in the total degradation products may be optimized. Alternatively, the microwave reactor may include an internal mixer which may be in the form of a rotating blade or fin. The mixer may be made from stainless steel or other suitable material. Preferably, the mixer is interactive with the electromagnetic radiation to vary the electromagnetic distribution pattern within the chamber.
Preferably, the chamber is tubular with rounded ends. The absence of sharp comers in the curved interior surface reduces the likelihood of arcing or xe2x80x9chot spotsxe2x80x9d. Furthermore, such a configuration facilitates the calculation of modes (filed configurations) within the chamber.
The microwave reactor includes an outlet through which volatile degradation products can be removed. Typically, the chamber is connected to a vacuum pump to facilitate removal of volatile products. The vacuum environment prevents burning (oxidising) and under our vacuum system, the pollution potential is totally eliminated. The reactor typically further includes means for condensing and isolating the volatile product.
The reactor may also include a temperature sensor at several locations to monitor the internal temperature, the temperature of the product and extract the volatiles from that product by identifying the temperature signatures of each component, assisting us to evaporate and condense each component.
Typically, the reactor further includes an inert gas inlet to introduce inert gas into the chamber if needed.
The reactor includes an inlet for introducing microwave radiation into the chamber. Typically the microwave inlet is covered by a plate on which several waveguide terminations are provided. Each waveguide termination is connected to a respective source of microwave energy, such as a magnetron, via a transmission waveguides. Each waveguide termination is located over a respective aperture in the plate, but a ceramic window pressure plate may be placed across the waveguides interface to preserve the sealed nature of the reactor, The number and power rating of the magnetrons can be varied to suit the particular application. Preferably, the number of modes from each magnetron is relatively high. Due to the large number of modes and magnetrons, a relatively uniform energy pattern may be obtained in the reactor.
An outlet is provided for removal of solid by-products which is typically carbon. Generally, the outlet is located on a lower floor of the chamber and is closed by a retractable sliding door. A conveyor may be located below the outlet.
Although rubber absorbs microwave energy it is a poor conductor. This can lead to a relative slow rise in temperature, uneven heating and in some cases overheating and combustion. For example, when a ground sample of an untreated rubber from a used tire was subjected to microwave radiation, that 2450 MHz the temperature rise was observed to be about 6.2xc2x0 C./s for tire tread and about 2.1xc2x0 C./s for the side wall of the tire. This slow temperature increase can result in long microwave residence times and relatively large amounts of microwave energy required to carbonize the rubber. This can lead to an increase in costs. Whilst not wishing to be bound by theory it is believed that at the carbonization phase, the introduced chemical and oil components of the rubber (which made the rubber polar) created a dipole moment which assists in the rise in temperature by easier acceleration of molecules within the rubber generated by the microwaves. For economical reasons it is of course desirable that the value of the input energy is less than the value of the final products.
The present inventor has surprisingly and unexpectedly discovered that an elastomeric material subjected to the softening process of the present invention has an increased temperature rise when subjected to microwave radiation as compared to untreated rubber. It has been observed that irradiation at a microwave frequency of 2450 MHz the temperature rise of tire rubber can be up to about 36.9xc2x0 C./s. Whilst not wishing to be bound by theory, it is believed that trace amounts of the oil used in the softening solution facilitates conduction of the microwave energy through the elastomeric material.
The present inventor has also surprisingly and unexpectedly discovered that the effectiveness of the microwave heating step can be improved by pre-heating the elastomeric material to before prior to microwave radiation. Typically, the elastomeric material is pre-heated to between about 80 to about 150xc2x0 C., preferably between about 90 to about 120xc2x0 C. At this temperature, residual fluids used in the softening step will normally volatilize. Typically, the volatilized products are exhausted, from the heating zone, separated if desired and isolated. The isolated products may be recycled to the softening step. Alternatively, these chemicals may be subjected to further separation and for use in other applications.
According to a further broad from of the invention there is provided a method of carbonizing an elastomeric material, the method including preheating the elastomeric material to a temperature below which carbonization is initiated before heating the pre-heated material by exposing the material to microwave radiation to a temperature at which carbonization occurs.
Typically, the elastomeric material is heated in the microwave chamber to temperatures between about 900 to about 3000xc2x0 C., typically about 1300xc2x0 C. The elastomeric material is carbonized to produce a carbon product together with volatilized products and gaseous products. These products are removed from the chamber under vacuum and isolated. Sulfur and zinc oxides are vaporised and condensed into solids.
It has been observed that the carbon product obtained has a high carbon content, typically above 90%. If desired, the carbon product may be further purified by acid washing.